The instant invention relates to generally to useful cathode active materials for fuel cells, more specifically to their use as the cathode material for Ovonic instant startup alkaline fuel cells. These inventive cathodes open up a tremendous number of degrees of freedom in fuel cell design by utilizing reduction/oxidation (redox) couples, such as metal/oxide couples, or simply couples which provide electrochemical oxidizer, preferably oxygen, to the fuel cell electrolyte for electrochemical xe2x80x9ccombustionxe2x80x9d. These redox couples, due to their electrochemical potential, provide the fuel cells employing them with an increased operating voltage that is adjustable by varying the redox couple used. Additionally the redox couple provide the fuel cell with the ability to store oxidizer within the electrode which not only provides for instant startup, but also provides the capability to provide short surge bursts of energy during demand surges and also allows for recapture of regenerative energy.
The instant application for the first time provides oxygen electrodes, and fuel cells using such electrodes, which use oxide couples to yield a wide selection of operating voltages. Specifically, the instant inventors have determined materials, which used in combination with hydrogen-side electrodes, particularly with those constructed of Ovonic (Trademark of Energy Conversion Devices, Inc.) hydrogen storage material, both of which, in combination, yield high performance fuel cells having hydrogen storage capacity within the hydrogen electrode and oxygen electrodes which take advantage of low-cost, in comparison with the traditional platinum electrodes, oxide couples which allow selection of specific ranges of operating voltage of the electrochemical cells with a broad operating temperature range and the opportunity to provide instant-start by use of the hydrogen storage capability of the short-range order available in the material of the Ovonic hydrogen electrode.
As the world""s human population expands, greater amounts of energy are necessary to provide the economic growth all nations desire. The traditional sources of energy are the fossil fuels which, when consumed, create significant amounts of carbon dioxide as well as other more immediately toxic materials including carbon monoxide, sulfur oxides, and nitrogen oxides. Increasing atmospheric concentrations of carbon dioxide are warming the earth; creating the ugly specter of global climatic changes. Energy-producing devices which do not contribute to such difficulties are needed now.
A fuel cell is an energy-conversion device that directly converts the energy of a supplied gas into an electric energy. Highly efficient fuel cells employing hydrogen, particularly with their simple combustion product of water, would seem an ideal alternative to current typical power generations means. Researchers have been actively studying such devices to utilize the fuel cell""s potential high energy-generation efficiency.
The base unit of the fuel cell is a cell having an oxygen electrode, an hydrogen electrode, and an appropriate electrolyte. Fuel cells have many potential applications such as supplying power for transportation vehicles, replacing steam turbines and power supply applications of all sorts. Despite their seeming simplicity, many problems have prevented the widespread usage of fuel cells.
Presently most of the fuel cell R and D is focused on P.E.M. (Proton Exchange Membrane) fuel cells. Unfortunately, the P.E.M. fuel cell suffers from relatively low conversion efficiency and has many other disadvantages. For instance, the membrane and the electrolyte for the system is acidic. Thus, noble metal catalysts are the only useful active materials for the electrodes of the system. Unfortunately, not only are the noble metals costly, they are also susceptible to poisoning by many gases, specifically carbon monoxide (CO). Also, because of the acidic nature of the P.E.M fuel cell electrolyte, the remainder of the materials of construction of the fuel cell need to be compatible with such an environment, which again adds to the cost thereof. The proton exchange membrane itself is quite expensive, and because of it""s low proton conductivity at temperatures below 80xc2x0 C., inherently limits the power performance and operational temperature range of the P.E.M. fuel cell as the PEM is nearly non-functional at low temperatures. Also, the membrane is sensitive to high temperatures, and begins to soften at 120xc2x0 C. The membrane""s conductivity depends on water and dries out at higher temperatures, thus causing cell failure. Therefore, there are many disadvantages to the P.E.M. fuel cell which make it somewhat undesirable for commercial/consumer use.
The conventional alkaline fuel cell has some advantages over P.E.M. fuels cells in that they have higher operating efficiencies, they use less costly materials of construction, and they have no need for expensive membranes. The alkaline fuel cell also has relatively higher ionic conductivity in the electrolyte, therefore it has a much higher power capability. While the conventional alkaline fuel cell is less sensitive to temperature than the PEM fuel cell, the platinum active materials of conventional alkaline fuel cell electrodes become very inefficient at low temperatures. Unfortunately, conventional alkaline fuel cells still suffer from their own disadvantages.
For example, conventional alkaline fuel cells still use expensive noble metal catalysts in both electrodes, which, as in the P.E.M. fuel cell, are susceptible to gaseous contaminant poisoning. The conventional alkaline fuel cell is also susceptible to the formation of carbonates from CO2 produced by oxidation of the hydrogen electrode carbon substrates or introduced via impurities in the fuel and air used at the electrodes. This carbonate formation clogs the electrolyte/electrode surface and reduces/eliminates the activity thereof. The invention described herein eliminates this problem from the hydrogen electrode.
Fuel cells, like batteries, operate by utilizing electrochemical reactions. Unlike a battery, in which chemical energy is stored within the cell, fuel cells generally are supplied with reactants from outside the cell. Barring failure of the electrodes, as long as the fuel, preferably hydrogen, and oxidant, typically air or oxygen, are supplied and the reaction products are removed, the cell continues to operate.
Fuel cells offer a number of important advantages over internal combustion engine or generator systems. These include relatively high efficiency, environmentally clean operation especially when utilizing hydrogen as a fuel, high reliability, few moving parts, and quiet operation. Fuel cells potentially are more efficient than other conventional power sources based upon the Carnot cycle.
The major components of a typical fuel cell are the hydrogen electrode for hydrogen oxidation and the oxygen electrode for oxygen reduction, both being positioned in a cell containing an electrolyte (such as an alkaline electrolytic solution). Typically, the reactants, such as hydrogen and oxygen, are respectively fed through a porous hydrogen electrode and oxygen electrode and brought into surface contact with the electrolytic solution. The particular materials utilized for the oxygen electrode and hydrogen electrode are important since they must act as efficient catalysts for the reactions taking place.
In an alkaline fuel cell, the reaction at the hydrogen electrode occurs between the hydrogen fuel and hydroxyl ions (OHxe2x88x92) present in the electrolyte, which react to form water and release electrons:
H2+2OHxe2x88x92xe2x86x922H2O+2exe2x88x92(E0=xe2x88x920.828 v).
At the oxygen electrode, the oxygen, water, and electrons react in the presence of the oxygen electrode catalyst to reduce the oxygen and form hydroxyl ions (OHxe2x88x92):
O2+2H2O+4exe2x88x92xe2x86x924OHxe2x88x92(E0=xe2x88x920.401 v).
The total reaction, therefore, is:
2H2+O2xe2x86x922H2O(E0=xe2x88x921.229 v)
The flow of electrons is utilized to provide electrical energy for a load externally connected to the hydrogen electrode and oxygen electrode.
It should be noted that the hydrogen electrode catalyst of the alkaline fuel cell is required to do more than catalyze the reaction of H+ ions with OHxe2x88x92 ions to produce water. Initially the hydrogen electrode must catalyze and accelerate the formation of H+ ions and exe2x88x92 from H2. This occurs via formation of atomic hydrogen from molecular hydrogen. The overall reaction may be simplified and presented (where M is the catalyst) as:
M+H2xe2x86x922M . . . Hxe2x86x92M+2H+2exe2x88x92.
Thus the hydrogen electrode catalyst must not only efficiently catalyze the electrochemical reaction for formation of water at the electrolyte interface but must also efficiently dissociate molecular hydrogen into atomic hydrogen. Using conventional anode material, the dissociated hydrogen is transitional and the hydrogen atoms can easily recombine to form hydrogen if they are not used very efficiently in the oxidation reaction. With the hydrogen storage hydrogen electrode materials of the inventive instant startup fuel cells, hydrogen is stored in hydride form as soon as they are created, and then are used as needed to provide power.
In addition to being catalytically efficient on both interfaces, the catalytic material must be resistant to corrosion by the alkaline electrolyte. Without such corrosion resistance, the electrode would quickly succumb to the harsh environment and the cell would quickly lose efficiency and die.
One prior art fuel cell hydrogen electrode catalyst is platinum. Platinum, despite its good catalytic properties, is not very suitable for wide scale commercial use as a catalyst for fuel cell hydrogen electrodes, because of its very high cost resulting from the limited world supply. Also, noble metal catalysts like platinum, cannot withstand contamination by impurities normally contained in the hydrogen fuel stream. These impurities can include carbon monoxide which may be present in hydrogen fuel or contaminants contained in the electrolyte such as the impurities normally contained in untreated water including calcium, magnesium, iron, and copper.
The above contaminants can cause what is commonly referred to as a xe2x80x9cpoisoningxe2x80x9d effect. Poisoning occurs where the catalytically active sites of the material become inactivated by poisonous species invariably contained in the fuel cell. Once the catalytically active sites are inactivated, they are no longer available for acting as catalysts for efficient hydrogen oxidation reaction at the hydrogen electrode. The catalytic efficiency of the hydrogen electrode therefore is reduced since the overall number of available catalytically active sites is significantly lowered by poisoning. In addition, the decrease in catalytic activity results in increased over-voltage at the hydrogen electrode and hence the cell is much less efficient adding significantly to the operating costs. Overvoltage is the difference between the actual working electrode potential under some conditions and it""s equilibrium value, the physical meaning of overvoltage is the voltage required to overcome the resistance to the passage of current at the surface of the hydrogen electrode (charge transfer resistance). The overvoltage represents an undesirable energy loss which adds to the operating costs of the fuel cell.
In related work, U.S. Pat. No. 4,623,597 (xe2x80x9cthe ""597 patentxe2x80x9d) and others in it""s lineage, the disclosure of which is hereby incorporated by reference, one of the present inventors, Stanford R. Ovshinsky, described disordered multi-component multi-phase hydrogen storage materials for use as negative electrodes in electrochemical cells for the first time. In this patent, Ovshinsky describes how disordered materials can be tailor made (i.e., atomically engineered) to greatly increase hydrogen storage and reversibility characteristics. Such disordered materials are amorphous, microcrystalline, intermediate range order, and/or polycrystalline (lacking long range compositional order) wherein the polycrystalline material includes topological, compositional, translational, and positional modification and disorder. The framework of active materials of these disordered materials consist of a host matrix of one or more elements and modifiers incorporated into this host matrix. The modifiers enhance the disorder of the resulting materials and thus create a greater number and spectrum of catalytically active sites and hydrogen storage sites.
The disordered electrode materials of the ""597 patent were formed from lightweight, low cost elements by any number of techniques, which assured formation of primarily non-equilibrium metastable phases resulting in the high energy and power densities and low cost. The resulting low cost, high energy density disordered material allowed the batteries to be utilized most advantageously as secondary batteries, but also as primary batteries.
Tailoring of the local structural and chemical order of the materials of the ""597 patent was of great importance to achieve the desired characteristics. The improved characteristics of the hydrogen electrodes of the ""597 patent were accomplished by manipulating the local chemical order and hence the local structural order by the incorporation of selected modifier elements into a host matrix to create a desired disordered material. Disorder permits degrees of freedom, both of type and of number, within a material, which are unavailable in conventional materials. These degrees of freedom dramatically change a materials physical, structural, chemical and electronic environment. The disordered material of the ""597 patent have desired electronic configurations which result in a large number of active sites. The nature and number of storage sites were designed independently from the catalytically active sites.
Multiorbital modifiers, for example transition elements, provided a greatly increased number of storage sites due to various bonding configurations available, thus resulting in an increase in energy density. The technique of modification especially provides non-equilibrium materials having varying degrees of disorder provided unique bonding configurations, orbital overlap and hence a spectrum of bonding sites. Due to the different degrees of orbital overlap and the disordered structure, an insignificant amount of structural rearrangement occurs during charge/discharge cycles or rest periods there between resulting in long cycle and shelf life.
The improved battery of the ""597 patent included electrode materials having tailor-made local chemical environments which were designed to yield high electrochemical charging and discharging efficiency and high electrical charge output. The manipulation of the local chemical environment of the materials was made possible by utilization of a host matrix which could, in accordance with the ""597 patent, be chemically modified with other elements to create a greatly increased density of electro-catalytically active sites and hydrogen storage sites.
The disordered materials of the ""597 patent were designed to have unusual electronic configurations, which resulted from the varying 3-dimensional interactions of constituent atoms and their various orbitals. The disorder came from compositional, positional and translational relationships of atoms. Selected elements were utilized to further modify the disorder by their interaction with these orbitals so as to create the desired local chemical environments.
The internal topology that was generated by these configurations also allowed for selective diffusion of atoms and ions. The invention that was described in the ""597 patent made these materials ideal for the specified use since one could independently control the type and number of catalytically active and storage sites. All of the aforementioned properties made not only an important quantitative difference, but qualitatively changed the materials so that unique new materials ensued.
Disorder can be of an atomic nature in the form of compositional or configurational disorder provided throughout the bulk of the material or in numerous regions of the material. The disorder also can be introduced by creating microscopic phases within the material which mimic the compositional or configurational disorder at the atomic level by virtue of the relationship of one phase to another. For example, disordered materials can be created by introducing microscopic regions of a different kind or kinds of crystalline phases, or by introducing regions of an amorphous phase or phases, regions of an amorphous phase or phases in addition to regions of a crystalline phase or phases. The interfaces between these various phases can provide surfaces which are rich in local chemical environments which provide numerous desirable sites for electrochemical hydrogen storage.
These same principles can be applied within a single structural phase. For example, compositional disorder is introduced into the material which can radically alter the material in a planned manner to achieve important improved and unique results, using the Ovshinsky principles of disorder-on an atomic or microscopic scale.
Additionally, in copending U.S. application Ser. No. 09/524,116, (""116), the disclosure of which is hereby incorporated by reference, Ovshinsky has employed the principles of atomic engineering to tailor materials which uniquely and dramatically advance the fuel cell art. The invention of ""116 application has met a need for materials which allow fuel cells to startup instantaneously by providing an internal source of fuel, to operate in a wide range of ambient temperatures to which a fuel cell will be exposed to under ordinary consumer use and to allow the fuel cell to be run in reverse as an electrolyzer thereby utilizing/storing recaptured energy. The hydrogen electrodes of the ""116 fuel cells are formed from relatively inexpensive hydrogen storage materials which are highly catalytic to the dissociation of molecular hydrogen and the formation of water from hydrogen and hydroxyl ions as well as being corrosion resistant to the electrolyte, resistant to contaminant poisoning from the reactant stream and capable of working in a wide temperature range.
The next step in the evolution of the fuel cell would be to find suitable materials to replace the expensive platinum oxygen electrode catalysts of conventional fuel cells. It would also be advantageous to provide the oxygen electrode with the ability to store chemical energy (possibly in the form of chemically bound oxygen) to assist in the instant startup of the fuel cell as well as recapture energy Thus there is a need within the art for such a material. The invention described in this application is significant in that it provides the next step in the development of such electrochemical cells. With this invention, the oxygen electrode can be selected from a broad menu of available possible redox couples. These redox couples in addition to providing a store of chemical energy, allow the operating voltage of the fuel cell to be selected, by judicious choice of the redox couple used.
The object of the instant invention is a fuel cell which has the ability to start up instantly and can accept recaptured energy such as that of regenerative braking by operating in reverse as an electrolyzer. The instant startup fuel cells have increased efficiency and power availability (higher voltage and current) and a dramatic improvement in operating temperature range (xe2x88x9220 to 150 xc2x0 C.) The fuel cells of the instant invention also have additional degrees of freedom over the fuel cells of the prior art in that the voltage output of the cell can be tailored and they are capable of storing regenerated energy.
The oxygen electrodes of the instant fuel cells operate through the mechanism of redox reactions which uniquely provide multiple degrees of freedom in selecting the operating voltages available for such fuel cells. Such oxygen electrodes provide the fuel cells in which they are used, particularly alkaline fuel cells, with a level of chemical energy storage within the oxygen electrode itself. This means that such fuel cells will have a xe2x80x9cbufferxe2x80x9d or xe2x80x9cchargexe2x80x9d available within the oxygen electrode at pre-startup.
The fuel cell oxygen electrode comprises an active material capable of reversibly storing energy through the mechanism of a redox couple. The electrode has a first surface region situated to be exposed to molecular oxygen which includes a catalytically acting component promoting the absorption of oxygen through said first surface region and into said active material to chemically charge said active material through oxygen absorption.
The fuel cell oxygen electrodes of this invention may utilize redox couples, particularly metal/oxides couples selected from the group of metals consisting of copper, silver, zinc, cobalt and cadmium. Another useful redox couple is the nickel hydroxide/nickel oxyhydroxide couple disclosed herein above.
The fuel cell also employs an anode active material which has hydrogen storage capacity. The anode active material is a hydrogen storage alloy which has excellent catalytic activity for the formation of atomic hydrogen from molecular hydrogen, outstanding catalytic activity toward the formation of water from hydrogen ions and hydroxyl ions, and has exceptional corrosion resistance toward the alkaline electrolyte of an alkaline fuel cell. The anode active material is also low cost, containing no noble metals. The materials are robust and poison resistant. The electrodes are easy to produce, by proven low cost production techniques. The hydrogen electrode eliminates the use of carbon therein, thus helping to eliminating the carbonate poisoning of the fuel cell.
The anode active hydrogen storage alloys useful in the instant startup fuel cells reversibly absorbs and releases hydrogen and has a fast hydrogenation reaction rate and a long shelf-life. The hydrogen storage alloy is preferably selected from Alkaline Earth-Nickel alloys, Rare-Earth/Misch metal alloys, zirconium alloys, titanium alloys and mixtures or alloys thereof. The preferred hydrogen storage alloy contains enriched catalytic nickel regions distributed throughout the oxide surface of the particulate thereof. The catalytic nickel regions are 50-70 Angstroms in diameter and vary in proximity from 2-300 Angstroms (preferably from 50-100 Angstroms). An example of such an alloy has the following composition:
(Base Alloy)aCobMncFedSne
where the Base Alloy comprises 0.1 to 60 atomic percent Ti, 0.1 to 40 atomic percent Zr, 0 to 60 atomic percent V, 0.1 to 57 atomic percent Ni, and 0 to 56 atomic percent Cr; b is 0 to 7.5 atomic percent; c is 13 to 17 atomic percent; d is 0 to 3.5 atomic percent; e is 0 to 1.5 atomic percent; and a+b+c+d+e=100 atomic percent.